Scientific Method —

No absolutes: How shifting plates completely remake the Earth

Tracking geologic hotspots shows a wobbly Earth in constant flux.

Plate tectonics is one of the most successful theories in the history of science. Beyond its scientific successes, it's widely accepted by the public, since it explains a lot about the world that we see around us.

But like other successful theories, it has its share of awkward inconsistencies. A recent paper in the Journal of Geophysical Research attempted to tackle one of these inconsistencies—finding an absolute reference frame for the movement of the plates—but failed so badly that its authors advise other scientists not to even bother trying. But as part of their failure, they came up with a new measure of one of the more unexpected consequences of plate tectonics.

All of plate tectonics is driven by density differences in the material beneath our planet's solid surface. These drive the shifting plates, power hot-spot volcanoes, and recycle material to the planet's surface. They also make sure that the mass of the Earth is never evenly distributed. As that mass shifts internally, it actually causes the Earth's spin to wobble around a bit. As a result, even the Earth's axis of rotation doesn't provide an absolute reference frame. In the process of failing to find an absolute reference frame, though, the authors have provided a detailed map of how the Earth's true pole has wandered over millions of years.

Making the numbers add up

One of the larger successes of plate tectonics is that it offers an explanation for island chains. Groups of volcanic islands, like the Hawaiian islands, can trace straight lines for thousands of miles if one considers the largely submerged remains of former islands. Plate tectonics offers an explanation: there are stationary hot spots in the mantle that drive volcanic eruptions. The plates simply slide over a hot spot, which builds volcanoes that later become inactive and erode as they slide past.

This process is so regular that it's one of the ways that scientists have tracked past plate motion. For the Hawaiian chain, it's even possible to see a sudden left turn, as the Pacific plate changed its direction of motion. These measurements tend to line up well with others based on measurements made at the boundary of plates.

So, stationary hot spots, moving plates. That would make hot spots a great reference frame for plate movement. Just pick absolute locations for the hot spots, then you could track the entire planet's plates as they slid across them. Just one small problem: it doesn't work. "It was soon realized," the authors write, "that a reference frame defined by fixed hot spots from the Pacific Ocean could not adequately reproduce hot spot tracks in the Indian and Atlantic Ocean."

In other words, although a hot spot appears to be a fixed reference frame for a given plate and its neighbors, our best data indicates that different hot spots appear to be moving relative to each other.

But our best data is an ever-changing thing, and the authors decided it was time for another try. They went through the literature and pulled out any information they could find about rates and directions of plate motion, and integrated it all into a single model. Despite several iterations that made for a progressively better fit to the data from individual hotspots, there was no way to get things to work out globally. "Our attempts to define a global fixed hot spot reference frame have failed to produce acceptable fits to the segments of hot spot tracks formed from Late Cretaceous to Paleogene time (80–50 Ma [million years])," the authors concede.

Their conclusion? It's time to give up on the idea of hot spots being fixed. If we're ever going to have an absolute reference frame, it's not going to come from hot spots.

Shifting references on an unstable Earth

Based on the paper, though, it's hard to tell what else might provide an absolute reference. The authors' work provides further evidence that the entire surface of the Earth (the lithosphere) is moving relative to its interior, the mantle. In other words, the surface of the Earth is not completely coupled to the core, something that had previously been suggested to be the case for Saturn's moon Titan. In the case of Earth, the so-called "lithosphere rotation" involves a slow drift westward, shifting about a tenth of a degree every million years on average. However, the rate isn't even, and has been nearly three times that at some point in the past, apparently at the time when the Indian plate was accelerating toward Asia.

It isn't just that we lack a fixed reference frame to track the plates. It's that plate tectonics itself shifts such enormous masses around that these skew the reference frame.

As it turns out, this also wipes out another potential reference frame, the axis of the Earth's rotation. If the Earth were a uniform, solid sphere, its axis of rotation would remain stable. But the whole idea behind plate tectonics is that the mass isn't distributed evenly. The hot spots at issue here push to the surface of the crust precisely because they're hotter and thus less dense than the surrounding material. On larger scales, it's this density-driven convection that powers the shifting of the continents themselves. As crust is driven into the Earth's interior at subduction zones, it places sheets of solid material deep under the crust that take millions of years to come to equilibrium with their new surroundings.

So, not only is the Earth not uniform, but its internal differences are constantly shifting around. All of which feeds back into the dynamics of its rotation, leading to a phenomenon called "true polar wander"—its axis of rotation hasn't always run through the sites of the current North and South Poles. In fact, during the period from 90 million to 40 million years ago, the poles drifted nearly 10 degrees and then snapped back.

Technicalities and the big picture

The paper itself is long, dry, and very technical; it's not the sort of thing that I'd recommend anyone read unless they're actively working in this field. But the ideas within it are important and compelling.

One important idea is that even our most successful scientific theories are filled with enough discrepancies and inconsistencies to keep scientists gainfully employed for generations. But it's important to keep these in perspective. Not knowing something, or even getting it wrong, doesn't mean that we don't know anything, or that every little inconsistency means we should throw the entire structure out.

The story of plate tectonics itself highlights how oddities on their own aren't enough to overthrow a dominant idea. Instead, you have to come up with something that explains not only the discrepancies, but everything else that the dominant idea gets right. Even then, it's not easy, something that is very clear from the reception that plate tectonics got when it was first suggested almost precisely 100 years ago.

One of the reasons that many people have a hard time accepting some aspects of science is that these ideas make them feel uncomfortable. Plate tectonics doesn't have the same emotional impact as the Copernican revolution, which told us that our place in the Universe wasn't special. But it does tell us that our place wasn't even a place for most of its history: continents shift, islands grow and vanish, and there are apparently no fixed frames of reference.

Without the energy brought to the Earth's surface by plate tectonics, however, it's not even clear that life itself would have been able to flourish, and it certainly wouldn't have evolved the way it has without the changing climates and landscapes.

Giving up a fixed frame of reference to have all that seems like a worthwhile tradeoff.